We have increased the understanding of the cell surface antigens of mouse sarcomas and the immune response to such antigens. We have proven that most cross-reactive antibodies seen in the sera from mice immunized with syngeneic or allogeneic chemically induced sarcomas are directed against antigenic determinants of gp70 and ;15(E) envelope proteins of endogeneous murine leukemia virus (MuLV). They have shown, furthermore, that the suppressor cell response to tumor antigens includes a nonspecific component, activated by an antigen-specific event. This has led us to now study whether some antigens, present on sarcoma cells, e.g., gp70 of MuLV, can induce a suppressor cell response inhibiting the development of effective tumor immunity also against individually unique, tumor specific transplantation antigens. Our attempts to raise hybridomas which define antigens unique to each methylcholanthrene (MCA) induced sarcoma (as the transplantation antigens appear to be) have not been successful, while hybridomas to viral antigens, such as gp70, can be raised easily. One can also raise hybridomas to certain other shared antigens, which we are now studying. The work which we have been pursuing on immune regulation may help us out towards analyzing the unique antigens as well; we have established a T-T hybridoma which makes a suppressor factor specific for the antigens of an MCA induced BALB/c sarcoma, MCA-1490. We shall now try to use the hybridoma product towards characterizing the antigens of that sarcoma. So far, we have demonstrated that this product has a receptor for antigens of MCA-1490 cells, which is not detectable on cells from other MCA-sarcomas or on normal BALB/c fibroblasts. The experience which we have originally got from studying mouse sarcomas, and the techniques first worked out with such, have been utilized towards an analysis of human tumors. This work has provided much knowledge about human tumor associated, cell surface antigens, detectable with the monoclonal antibody techniques.